D

Word

Meaning

Daphnis

Dark Dust Cloud

A cloud of dust which doesn't let light through! It is usually area in space which contains a concentration of gas and dust. As it prevents light from penetrating it, it means that stars behind it are not visible.

Dark Matter

Most of space is "dark matter"! It is matter (particles of gas and dust) that is too dim to be picked up by telescopes, but astronomers know it's there by the influence of its gravity (recognised by the effect it has on other objects).

Day

Length of time it takes for a planet to completely rotate on its axis, most often in reference to the length of time it takes for Earth to complete a rotation. A "day" on Mars often referred to as a "Sol".

Deimos

One of two of Mars' moons (Phobos is the other). Both moons are irregularly shaped with Deimos being the most distant and the smaller of the two moons, orbiting at 23,460 kilometres (15,577 miles) away from the planet and at a size of 16 km by 12 km (10 miles by 7.5 miles). Deimos was discovered on 12th August 1877 by American astronomy Asaph Hall Senior. He also discovered Phobos on the same date. Deimos takes 30.4 hours to orbit Mars.

Density

A measure of how closely packed matter is. Gas is less dense (less compact) than water. Water is less dense than a solid.

Desdemona

Moon of Uranus, discovered by Voyager 2 in 1986.

Despina

Moon of Neptune, third closest to the planet. Its diameter is 152 kilometres (94 miles) and it orbits at 52,526 km (32,638 miles) from Neptune. Despina was discovered in 1989 by the Voyager 2 space craft.

Diameter

The distance from one side of a round object to the opposite side going through its centre. In astronomy, this usually refers to the width of a planet, moon or star. Some of these objects are not completely spherical (like Jupiter) so the diameter is wider than the distance from top to bottom.

Dione

Double Planet

Two planets which are attracted to each other by their respective gravitational pulls. They orbit around a point in between each other (but not necessarily exactly in the middle of each other). An example of this in the Solar System may be Pluto and Charon and some think even Earth and the Moon. Also referred to as a Binary Planet.

Double Star

Two stars that are attracted to each other by their respective gravitational pulls. They orbit around a point in between each other. Also known as Binary Star